A typical cooling system of a conventional gas turbine is schematically shown in FIG. 4. The gas turbine includes an air compressor 1, a combustion section 3 and a turbine section as main components. Intermediate stage bleeds 7a, 7b, 7c from the air compressor 1 and partial compressor outlet air 9 are led to stationary blades of the turbine 5 so as to cool them. In addition, a portion of the outlet air of the air compressor 1 is led to blade roots 13 of rotor blades of the turbine 5 as a combustor casing bleed, thereby cooling the rotor blades 15. In FIG. 5, a conventional structure for cooling the rotor blades 15 is illustrated. In FIG. 5, a turbine rotor has turbine discs 17a, 17b, 17c, 17d which are arranged in line along the rotor axis in mesh engagement between coupling teeth on facing surfaces thereof and through which spindle bolts 19 extend, and the rotating blades 15a, 15b, 15c, 15d are mounted on outer peripheries of the turbine discs 17a, 17b, 17c, 17d. The combustor casing bleed 11 for cooling, which flows in through an opening 21 in the turbine rotor, flows in an axial direction through axial bores 23a.about.23c in the turbine discs 17a.about.17c and reaches blade root portions 13a.about.13d through radial bores. The bleed or compressed air which flows into internal cooling holes in the rotating blades 15a-15d through the blade root portions 13a-13d, cools the rotor blades 15a-15d from within and finally blows out into the main flow of combustion gas.
Though the technology of cooling a turbine section with such aforementioned bleed air from the compressor has provided adequate effects, there is no end to the need for increasing the output of the gas turbine and improving the efficiency thereof, and it has therefore been proposed to increase the inlet temperature for combustion gas of the gas turbine in order to meet such needs. In this proposal, it is extremely difficult to keep the temperature of the turbine rotor blades below an acceptable value by cooling them with conventional compressed air and hence it has been proposed to use steam as a cooling medium. However, it is not permissible to emit steam into a working gas as with the compressed air in the conventional art.
Accordingly, an object of the present invention is to provide a gas turbine rotor for steam cooling which has a structure suitable for cooling turbine rotor blades with steam.